Forgot to mention: don't decrease this parameter to a too low value or you'll get "free buffer wait"
events.
I think the best way is to set db_files to a value slighty higher than the number of files and
adjust db_file_simultaneous_writes to a value that set _db_block_write_batch to a value higher than
the current DBWR batch size ("physical writes"/"write requests" in v$sysstat).